In recent years, with improvement of architectural techniques and diversification of cement application methods, various additives for admixtures of a cement composition are being used according to the purpose of execution including the scale of the structure and the site of placement.
Of these, a retarder for retarding the setting time of the cement composition is widely used. The retarder is previously added to the cement composition or coated on the surface or formwork of concrete still in the unhardened state for various purposes, for example, for preventing a reduction in fluidity of the concrete between hydration and transportation/placement to thereby ensure the working time or for junction integrating adjacent joint parts in the placement process for constructing a large concrete structure by sequentially placing a plurality of concrete layers and/or concrete sections.
The retarder acts such that the active component thereof couples with calcium ion liberated by the initial hydration of cement, into a certain form (for example, formation of a complex compound) and this moiety covers the surface of a cement particle to thereby bring out the effect of retarding the setting of cement. Known examples of active components include hydroxycarboxylic acids or salts thereof such as gluconic acid, glucoheptonic acid, arabonic acid, malic acid, citric acid, tartaric acid, pyruvic acid and glycolic acid, saccharides (e.g., monosaccharides, disaccharides, trisaccharides, oligosaccharides, sugar-alcohols) such as saccharose, glucose, fluctose, galactose, mannose, lactose, raffinose, dextrin and sorbitol, and (hydrogen) carbonates of alkali metals, such as potassium carbonate and sodium carbonate. A retarder (composition) comprising these components individually or in combination of two or more thereof is being used or studied (see, for example, JP-A-54-60372 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-59-156950, JP-B-4-20866 (the term "JP-B" as used herein means an "examined Japanese patent publication")). In particular, a retarder having blended therein sodium gluconate as a hydroxycarboxylate is one of the most widely used retarders.
The amount of retarder added to the cement composition is usually from 0.1 to 0.7 wt % based on the weight of cement in practical use and thereby, a retardation time of from 1 to 24 hours can be achieved, though it may vary depending on the ambient conditions. The retarder is usually used by adjusting the addition amount according to the desired retardation time.
However, these conventional retarders have the following problems. For example, in the case of saccharose as a saccharide, great retardation may be obtained but the distribution thereof is large and the capability of controlling the retardation time is poor. In the case of sodium gluconate which is a hydroxycarboxylate and which is widely used, the retardation effect is accompanied with water reduction effect, accordingly, the water-cement ratio must be adjusted whenever it is used by taking account of the incidental water reduction effect as well as the adjustment of the addition amount depending on the desired retardation effect, thus, cumbersome operations are necessary. As described above, the amount of the conventional retarder added is in practice from 0.1 to 0.7 wt % based on the weight of cement. However, in view of strength of the cement composition after setting and profitability, it is preferred to achieve higher retardation effect with a smaller addition amount. Existing retarders, even a retarder composition having blended therein two or more active components, have not yet succeeded in solving the problems with respect to the capability of controlling the retardation time and the cumbersomeness the incidental occurrence of undesired water reduction effect which accompanies their use.
The retarder of a concrete composition must show excellent retardation effect with a smaller addition amount, exhibit superior capability of controlling the retardation time as a result of the adjustment of the addition amount, and be free of incidental occurrence of water reduction effect.